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  <title>Document</title>
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<body>
  <script>
    var averageWaitingTime = function (customers) {
      let sum = 0, t = 0
      for (let [start, cost] of customers) {
        t = Math.max(start, t)
        t += cost
        sum += t - start
      }
      return sum / customers.length
    };
    console.log(averageWaitingTime([[1, 2], [2, 5], [4, 3]]));
  </script>
</body>

</html>